1. Field of the Invention
The present invention relates to the use of nonsystematic repeat-accumulate codes in a communication system.
2. Description of Related Art
Wireless communication systems that use multiple antennas at the transmitter and multiple antennas at the receiver, so-called multi-input multi-output (MIMO) systems, can achieve dramatically improved capacity compared to single antenna systems, i.e., systems that have a single antenna at the transmitter and a single antenna at the receiver. In a general MIMO wireless communication system, a data stream to be transmitted is processed, using well-known techniques, to form space-time coded signals that are transmitted over a plurality of different transmit antennas.
The signals emanating from the transmit antennas arrive at receive antennas as a superposition of each of the transmitted signals. Though the transmitted signals interfere with each other, received signals are processed in the receiver to separate out and decode these super-positioned signals. The receiver typically uses a MIMO detector and channel decoder several times over in an effort to reduce the number of bit errors in the decoded signal.
The channel coding used to code signals of the data stream to form the space-time coded signals is used at the receiver for error correction. That is, the channel decoder at the receiver may be able to recover bits that arrive at the receiver in error, due to noise and/or interference. Conventionally, a strong channel code, such as a turbo code, has been used. However, research has shown that an increase in the strength of the channel code does not necessarily result in a reduction in the errors in the decoded signal at the receiver when the number of antennas at the transmitter is larger than the number of antennas at the receiver.
Co-pending and commonly assigned U.S. patent application Ser. No. 10/180,727, filed Jun. 26, 2002 and entitled “MIMO SYSTEMS HAVING A CHANNEL DECODER MATCHED TO A MIMO DETECTOR” describes the use of low density parity check codes (LDPC) codes as channel codes in a communication system where the receiver has a MIMO detector, particularly when the number of transmit antennas is larger then the number of receive antennas. LDPC codes can be adjusted to change decoding properties so that a LDPC code decoder at the receiver may be designed having a transfer characteristic curve that is matched to the transfer characteristic curve of a MIMO detector.
The transfer characteristic curve may be defined as a curve that shows extrinsic mutual information content of soft value bits (bits whose value conveys both whether the bit is a one or zero and an indication of the certainty with which this is known) at an output of a device (such as a channel decoder) determined as a function of the mutual information content of soft value bits at the input of the device. A transfer characteristic curve may be illustrated with extrinsic information transfer (EXIT) charts, bit error rate (BER) charts or signal to noise ratio (SNR) charts, as is known in the art.
Repeat-accumulate (RA) codes have recently been devised as a competitive alternative to parallel concatenated (PC or turbo) and LDPC codes. RA codes are encoded by using four types of operations, in the following order: variable rate repetition encoders, an interleaver, variable rate modulo-2 adders (single parity check encoders), and an accumulator. The general structure is described in a paper by H. Jin, et al., entitled “Irregular repeat-accumulate codes,” in Proc. 2nd Int. Symp. On Turbo Codes, Brest, France, 2000. In the context of multi-input, multi-output (MIMO) systems, both LDPC and RA codes can operate near capacity of the system. One type of irregular RA codes are known as nonsystematic repeat-accumulate (RA) codes. Nonsystematic RA codes are a developing family of iteratively decodable channel codes that can operate close to capacity for many channels.